It is about 4 months old. I am uncertain as to the exact timing of voice changing/ drake feathering. It seems to still have the ability to be loud when it wants to, however such episodes are becoming less and less frequent. The quacking is hard for me to gauge since we just recently started with ducks, so I have little basis for comparison. I was hoping someone would be fimiliar enough with the breed to make a good guess or maybe we could just put it to a vote?

At four months old you should be able to tell the difference in the quack...the boys are raspy, almost like a loud duck whisper. The girls actually quack. I could tell my Indian runner drake (that was supposed to be a hen) was a drake at 2 months due to his voice. If your duck quacks loudly, chances are she is a she.

Now they're 3.5 months and the drake has his beautiful green sheen (he's a black) and his curly tail feather.

Since I have three hens, I don't mind. The boys are quieter and it just means I could incubate duck eggs if I choose to.

different species have different time frames and everything lol each duck is unique in every way. My drake was 4.5 months old when he finally got his "Sex Feather". Hes a Buff/Orpington Drake.
His name is Cheeze his mates name is Quackerz.
Here is a pic of my male a female pair. Yes my drake is much much quieter than my hen lol
She is VERY LOUD!! Lol Hope this helps alittle.. Hopefully if you look close enough you can see his "Sex Feather"